The invention relates to a roll of controllable flexure, especially for machines for the production and processing of webs of paper or plastic.
Webs of paper or plastic are frequently pressed between two or more rolls in the course of their production or processing. In general, a uniform distribution of the pressure in the nips between the rolls is required during such pressing. However, inasmuch as the rolls undergo a perceptible sag due to their own weight and the high contact pressure, especially when the working width is very wide, the pressure at the ends of the rolls is greater than in the center. This leads to undesired effects in the manufacture and processing of the webs.
Rolls of controllable flexure are basically known. They generally have a substantially cylindrical internal part and a tubular outer part which extends over a portion of the length of the internal part, surrounding the latter, and is mounted on the inner part at both of its extremities for rotation about the cylinder axis. Devices are generally disposed between the inner part and the outer part for the production of compulsive forces acting radially between the inner part and the outer part.
Thus, in a known roll of controllable flexure the outer part is floatingly mounted on the inner part, the space between the outer and inner parts being divided into two chambers extending longitudinally, which contain a hydraulic fluid which in one of the two chambers is placed under elevated pressure. This elevated pressure is intended to produce an upward arching in the middle of the roll to compensate the sag produced by the roll's own weight. This known roll, however, is expensive to make on account of the fluid feed lines and the sealing problems, and the control of its sag is relatively poor.
In another known roll of controllable flexure, which likewise operates on the hydraulic principle, a plurality of supporting elements are disposed along a generatrix on the inner part upon which the outer part is directly supported on a film of lubricant. The supporting means can be forced radially outward by hydraulic pressure and thereby can produce the desired additional compulsive forces between the inner part and the outer part. This known roll is very expensive to make, because each individual source of support must be connected by high-pressure lines to the hydraulic circuit. Considerable sealing problems result, as well as difficulties in the assembly and maintenance of the roll.